Exactly two things can cause a mismatch with an image and a preview...
Both are all about cache.
You upload a file...but use the same preview name as a previous file...and the site cache reads it as the same and the same preview image is posted to both files...fault there is with the namer/uploader of the file. When it's spotted an email is usually sent...along the lines of....
"Unfortunately it failed to upload. The preview needs to be a full-size jpg image ...the small image is generated by the site. ...and the name must be unique to you...and only alpha-numeric." [that's a cut/paste from my 'list']
The other possibility is with they uploader's subsequent checking of the result....but getting the wrong image 'returned' because of what is STILL in their browser cache....that, too is not a fault of the site or it's moderators.
The only sure-fire way to guarantee no confusion/error would be any original upload be deleted from the server...[as I suggested in #1], or for the file AND preview to be uploaded with new unique names...as Zoomba suggested in #9.
The rest of this is a reflection of the unwarranted tone inherent in the thread title, along with the assertion that the fault was/is elsewhere, and to paraphrase - "your wrong" [sic]....
A simple polite request for assistance from the site's volunteer moderators would have been more appropriate.
Either way it was resolved....